A reed valve which acts as a stopper valve has been used in an air intake passage of a 2-cycle engine. Such a reed valve is preferably used because of its simple structure and minimal leakage of air-fuel mixture.
FIG. 6 is a sectional end view illustrating an engine containing an existing reed valve, wherein reference numeral 1 represents an air intake passage. The air intake passage 1 communicates with a crank chamber 3 which in turn communicates with a combustion chamber or cylinder 2. Two valve seats 4, which each have a frame-like shape and face the crank chamber 3, are disposed in a V-shaped manner at the lower end portion of the air intake passage 1. Reed valves 5 are each secured to one side end of a respective one of the valve seats 4 and are arranged to oppose each other. A respective stopper member 6 is, as illustrated, secured to one end of each reed valve 5 in a manner yielding a somewhat laminated arrangement so that this stopper member 6 regulates the lift (degree of opening) of the reed valve 5.
A carburetor (omitted from illustration) supplies an air-fuel mixture to a location upstream of the reed valves 5 in the aforesaid air intake passage 1, while an exhaust port 7, a scavenge port 8 and an intake port 9 which communicates with the air intake passage 1 are bored in the cylinder 2, which ultimately receives the air-fuel mixture. These ports are opened and closed by reciprocation of an opening piston shown in broken lines at 10. That is, at a lower dead center position of the piston 10, the exhaust port 7 and the scavenge port 8 open, while at a top dead center position, the intake port 9 opens. Reference numeral 11 represents a crank shaft, reference numeral 12 represents a crank wheel and reference numeral 13 represents a crank pin.
When the piston 10 rises, the air pressure in the crank chamber 3 becomes negative, whereby the free end of the reed valves 5 open so as to take the air-fuel mixture into the crank chamber 3. A part of the air-fuel mixture is taken into the crank chamber 3 through the air intake port 9 at a position near the top dead center position of the piston 10. When the piston 10 is then lowered, the air pressure in the crank chamber 3 changes to positive, whereby the reed valves 5 are restored to their original closed positions so as to close the air intake passage 1, whereby the air-fuel mixture in the crank chamber 3 is compressed. When the piston 10 nears the lower dead center position, the scavenge port 8 opens, whereby the air-fuel mixture in the crank chamber 3 is pressed into the cylinder 2. During the next rising of the piston 10, the next quantity of the air-fuel mixture is taken into the crank chamber 3 while the air-fuel mixture already in the cylinder 2 is compressed and then sparked for burning, whereby the piston 10 is forcibly lowered so as to drive the crank shaft 1, to compress gases in the crank chamber 3, and to open the exhaust port 7 and exhaust the gases at a position adjacent to the lower dead center position of the piston just prior to the scavenging stroke.
The reed valve device acts to regulate the air intake passage 1 to open and close it in accordance with the reciprocation of the piston 10. The device, therefore, is an important factor for the air intake efficiency, which is the important factor for the engine power output when the quantity of the air-fuel mixture is too small or large. The size, shape, material and so forth of the valve are therefore properly designed in accordance with the response and resistance at the time of taking in air and so forth. However, the following problems between the relationship between the spring constant of the reed valve and the engine speed are raised.
FIG. 7 illustrates the relationship between the engine power output (vertical axis) and engine speed (lateral axis). The characteristics change as shown by the curve A (short dashed line) when a reed valve having a small spring constant is used, while they change as shown by curve B (alternate long and short dash line) when a reed valve having a relatively large spring constant is used. As can be clearly seen from this figure, the curve A shows a good response to the relatively slow flow of the air-fuel mixture in the low and lower intermediate speed ranges of the engine, whereby high power can be obtained. However, it shows a rapid reduction in output (jumping of the reed valve) at the point of entrance to the high rotational speed range, because of natural vibration. Although it eventually restores much of the output, it is not stable, and total output consequently decreases. On the other hand, a reed valve having a large spring constant can avoid the aforesaid lowering of output in the upper intermediate and high speed ranges because of natural vibration. However, since sufficient opening or lift cannot be obtained in the low and lower intermediate ranges, total output decreases as shown by the curve B.
In order to overcome the aforesaid problems, a device in which the spring constant of the reed valve is small at low and lower intermediate engine speeds and is large at upper intermediate and high engine speeds has been disclosed, for example in Japanese Patent Publication Nos. 36850/1971 and 40649/1983. According to these prior devices, although such problems can be solved, the mechanism for regulating the spring constant is complicated and large, or reliability in sealing accuracy of the reed valve and durability are not sufficient.
Thus, in such a reed valve device, the spring constant of the reed valve needs to be small in the low and intermediate engine speed ranges, while it needs to be large in the intermediate and high speed ranges. Some prior mechanisms can meet these requirements, but are difficult or complicated to adjust, or else the accuracy, durability and cost thereof are sometimes insufficient.
An object of the present invention is to provide for a 2-cycle engine a reed valve device capable of overcoming the aforesaid problems, in which a reed valve having a small spring constant is used, whereby it can respond from the low speed to high speed ranges for the purpose of optimizing the engine output.